"Breast-feeding mothers aren’t going to go out and get into bar fights, but if someone is threatening them or their infant, our research suggests they may be more likely to defend themselves in an aggressive manner," study researcher Jennifer Hahn-Holbrook, of the University of California in Los Angeles, said in a statement.

The study is small, and Hahn-Holbrook is currently planning to replicate her results in a larger group of women.

Facing fears

Hahn-Holbrook and her colleagues suggest the same mechanism that gives mother bears and other lactating animals the resolve to aggressively protect their young may also apply to humans.

The researchers found that breast-feeding moms showed lower blood pressures than formula-feeding moms and non-mothers did. This lowered state of anxiety (and fear) may, in turn, may give women the extra courage to protect themselves and their young. Supporting this connection, the researchers didn't see any increase in aggression in mothers who bottle-fed formula to their babies.

"Breast-feeding has many benefits for a baby’s health and immunity, but it seems to also have a little-known benefit for the mother," said Hahn-Holbrook. "It may be providing mothers with a buffer against the many stressors new moms face, while at the same time giving mothers an extra burst of courage if they need to defend themselves or their child."

How moms defend

The study followed three groups of women — 18 nursing mothers, 17 women who fed their babies formula, and 20 non-mothers. The women were separated from their infants (which were between 3 months and 6 months old) and asked to perform a computer task.

During the task they supposedly competed against a research assistant posing as an overtly rude study participant. (They were introduced to the rude assistant before the test, but actually competed against a programmed computer opponent, so as not to introduce error based on the opponent's actual ability.)

When they won, the victor's "prize" was the opportunity to press a button and blast a loud, lengthy burst of sound to the loser, an act of aggressiveness. Breast-feeding mothers delivered sound blasts more than twice as loud and long as those from formula-feeding mothers and non-mothers.

Stress and aggression

Before and during the experiment, the researchers measured the participants' stress levels (as indicated by their blood pressure). Before the experiment, breast-feeding mothers had lower blood pressures by about 5 points compared with other participants; they also had lower blood pressures during the stressful experiment, about 10 points lower than formula-feeding mothers and 12 points lower than non-mothers.

"Certainly, breast-feeding is not the only pathway through which maternal aggression may manifest, as many formula-feeding women I'm sure could attest to," Hahn-Holbrook told LiveScience in an email.

"Evolution seems to have used lactation in other species as one pathway to increase maternal aggression," Hahn-Holbrook said. "You have to remember that, before the advent of rubber nipples and formula, every mother would be breast-feeding her infant."

The study is detailed in the September issue of the journal Psychological Science.

You can follow LiveScience staff writer Jennifer Welsh on Twitter @microbelover. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

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Jennifer Welsh

Jennifer Welsh graduated from the University of California, Santa Cruz's Science Communication graduate program after working at a start up biotech company for three years after getting her Bachelor of Science in Biological Sciences from the University of Notre Dame. She has worked at WiredScience, The Scientist and Discover Magazine before joining the Live Science team.